A typical house arrest or home detention system includes a bracelet worn on a detainee's ankle. A radio transmitter in the bracelet transmits a coded signal received at a base station. Periodically, the detainee is instructed to place the bracelet near the base station unit. If the coded signal is not received, it is evident that the detainee is not at home. See U.S. Pat. No. 5,170,426 incorporated herein by this reference. Other “proximity detection” subsystems exist or have been proposed.
One potential problem with such prior systems is that the actual location of the detainee is not always known if the detainee has left the authorized area. So, those skilled in the art have proposed locater subsystems, such as GPS subsystems, to determine not only whether the detainee has left an authorized area, but also, if so, where the detainee is presently located. See U.S. Pat. No. 6,100,806 incorporated herein by this reference. GPS based subsystems, however, do not work indoors, and can be expensive.
Another use for proximity monitoring and locating systems includes people under the care of a caregiver such as Alzheimer's patients and children with autism. According to one study, about five million American's have Alzheimer's disease and 67% will likely wander during the course of the disease. See Sink, Kobinski, Newcomer & Yacki, “Ethnic Differences in the Prevalence and Pattern of Dementia—Related Behaviors,” The Journal of the American Geriatric Society, 2004, No. 52: pp. 1277-1283. Other people with disabilities, e.g., children with autism, also periodically wander away from their room, home, and/or caregiver.
Attempting to locate such people can be very difficult and a person can be severely harmed or expire before he or she is located if the locating effort, typically carried out with the help of law enforcement authorities, takes too long or is unsuccessful. Indeed, the period of time between the person leaving the area and detection of that fact is critical to a successful locating effort.